Question

It’s a simple opengl/sdl program. Generally theres a huge deque in which the program holds the display data for example a triangle will be inserted / readed in/from the dequeue like this ( GL_Begin …. Vertice 1 …. Vertice 2 … Vertice 3 … GL_End )

The Item struct

enum Shapes{
    LINE,
    POLYGON,
    TRIANGLE
};

struct Item{
    int id;
    int type;
    Shapes shape;
    double x;
    double y;
    double z;
};

…

void GEngine::CC2D(int id,double x,double y){ // Change Coordinates ... Move the Item

    for(int i=0;i<Items.size();i++){
        Item* item = &Items[i];                  // Pointer to the item INSIDE the deque because we need to edit it, we can not create a new instance of Item

        if(item->id == id && item->type == 2){
            item->x += x;
            item->y += y;
        }
    }

    DrawScene();
}

void GEngine::PollEvents( void (*Event)() ){
    int mbpressed = 0; // Mouse button pressed flag
    int mouse_xpre = 0;
    int mouse_ypre = 0;
    int move_id = 0; // TESTING
    while(1){
        while( SDL_PollEvent( &event ) ){

            switch( event.type ){
                case SDL_MOUSEMOTION:{
                    if(mbpressed == 1){
                        mouse_x = event.motion.x; // X2
                        mouse_y = event.motion.y; // Y2
                    //  (*Event)();

                    //  CC2D(  (   X2  -   X1     ),(   Y2  -   Y1     )
                        CC2D(move_id,(mouse_x-mouse_xpre),(mouse_y-mouse_ypre));      // The difference between the current and the previous mouse position is equal to the DX and DY which will be used to move the vertices.
                        mouse_xpre = mouse_x;     // X1
                        mouse_ypre = mouse_y;     // Y1

                        SDL_Delay(20);

                    }
                    break;

Now the issue. If i use CC2D(move_id,(mouse_x-mouse_xpre),(mouse_y-mouse_ypre)); it works fine but if I add the CC2D function into the Event one the algorithm fails and instead of the Item moving with the mouse it moves away from the screen.

In other words.

case SDL_MOUSEMOTION:{
                        if(mbpressed == 1){
                            mouse_x = event.motion.x; // X2
                            mouse_y = event.motion.y; // Y2
                        //  (*Event)();

                        //  CC2D(  (   X2  -   X1     ),(   Y2  -   Y1     )
                        CC2D(move_id,(mouse_x-mouse_xpre),(mouse_y-mouse_ypre));      // The difference between the current and the previous mouse position is equal to the DX and DY which will be used to move the vertices.
                        mouse_xpre = mouse_x;     // X1
                        mouse_ypre = mouse_y;     // Y1

                        SDL_Delay(20);

                    }
                    break;
}

^ Works just fine.

But

GEngine gEngine;

void Event(){
            gEngine.CC2D(0,(gEngine.mouse_x-gEngine.mouse_xpre),(gEngine.mouse_y-gEngine.mouse_ypre));
    }

int main(int argc, char *argv[]){

... code ...

gEngine.PollEvents(&Event);

return 0; // NEVER REACHED
}

with

case SDL_MOUSEMOTION:{
                    if(mbpressed == 1){
                        mouse_x = event.motion.x; // X2
                        mouse_y = event.motion.y; // Y2
                        (*Event)();

                        mouse_xpre = mouse_x;     // X1
                        mouse_ypre = mouse_y;     // Y1

                        SDL_Delay(20);

                    }
                    break;
}

does not …

To further simplify things:

Case SDL_MOUSEMOTION: ...... code ..... CC2D( params) ........

works fine but

Event(){ CC2D( params ) } // Main.cpp
Case SDL_MOUSEMOTION: ...... code ..... (*Event)() ........ // GEngine.cpp

fails to work as intended

Answer 1

You are declaring local variables here inside PollEvents:

int mouse_xpre = 0;
int mouse_ypre = 0;

Notice that they are named mouse_xpre and mouse_ypre. Then in the callback you are doing

gEngine.CC2D(0,(gEngine.mouse_x-gEngine.mouse_xpre),(gEngine.mouse_y-gEngine.mouse_ypre));
//                                      ^^^^^^^^^^                           ^^^^^^^^^^

which accesses member variables. You need to remove the declaration of the local variables so that they don’t hide the member variables and you’ll be using member variables in both places.